1. Field of the Invention
The invention relates to an ignition timing control apparatus and an ignition timing control method that control an ignition timing of an ignition plug according to an operating state of an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine (hereinafter, may be simply referred to as “engine”), an ignition timing control is executed to generate an output most efficiently using combustion, and to provide good exhaust gas purification performance and good fuel efficiency. It is known to be preferable to make a pressure in a combustion chamber reach a peak slightly after a compression top dead center, to generate the output most efficiently using energy generated by the combustion. Therefore, an ignition timing is set so that the pressure reaches the peak slightly after the compression top dead center. However, when the ignition timing is too early (i.e., when the ignition timing is too advanced), knocking occurs.
An ignition timing control apparatus, which executes a knock control system (hereinafter, referred to as “KCS”) control, is available. In the KCS control, it is determined whether knocking occurs using a knock sensor, and a feedback correction process (feedback control) is executed to adjust the ignition timing, according to the determination as to whether knocking occurs. Also, to execute the feedback correction process in quick response to the determination, a learning control is executed to store and retain, as a learning value, the degree of change in the ignition timing caused by the feedback correction process (for example, refer to Japanese Patent Application Publication No. 2002-031024 (JP-A-2002-031024) and Japanese Patent Application Publication No. 2005-147112 (JP-A-2005-147112)). The ignition timing control apparatus calculates a final ignition timing in the engine, for example, based on the equation (3) and the equation (4) described later (refer to, for example, FIG. 4).
In the ignition timing control apparatus described in Japanese Patent Application Publication No. 2002-031024 (JP-A-2002-031024), the learning value (KCS teaming value), which is learned by the earning control when the engine E is in a given operating state, is used in the ignition timing control executed when the engine E is in the other operating state. For example, a line indicating the ignition timing (estimated knocking occurrence ignition timing) is set to be parallel to a line for setting a most retarded ignition timing, and ignition is performed at the ignition timing on the line indicating the estimated knocking occurrence ignition timing. The estimated knocking occurrence ignition timing is the estimated most advanced ignition timing at which occurrence of knocking is suppressed. In this case, the line indicating the estimated knocking occurrence ignition timing is parallel to, and advanced with respect to the line indicating the most retarded ignition timing by the learning value that is used. Then, the final ignition timing is set based on the ignition timing feedback-corrected according to whether knocking occurs (refer to, for example, FIG. 4). As a result, the ignition timing when the engine is in the operating state is learned, and the learning value is updated. The learning value, which is learned in the above-described manner, is further used in the ignition timing control executed when the engine is in the other operating state.
In the above-described ignition timing control apparatus, to suppress occurrence of knocking, it is important to accurately set the estimated knocking occurrence ignition timing based on the most retarded ignition timing using the learning value learned by the learning control. That is, if the estimated knocking occurrence ignition timing cannot be accurately set, much time is required to stop knocking. Therefore, knocking may continue to occur a plurality of times. Also, knocking noise may be transmitted to a vehicle cabin, and an occupant may feel discomfort. Further the engine may be adversely influenced.
However, it is difficult to accurately set the estimated knocking occurrence ignition timing in individual engines, because there are various driving environments, and there are variations among individual engines. In addition, it may be difficult to accurately set the estimated knocking occurrence ignition timing, depending on an operating condition of the engine.
In such a case, if the learning value learned by the above-described learning control is used without correction, the estimated knocking occurrence ignition timing may greatly vary. Accordingly, when the engine is operated in a certain operating condition, it is not appropriate to use the learning value learned by the above-described learning control without correction.
For example, when a driver operates an accelerator pedal, and therefore, a point indicating the operating state of the engine moves into a low load region, it is not appropriate to use the learning value without correction. When the point indicating the operating state of the engine is in the low load region, the ignition timing is sensitive to the driving environment, the variations among the individual engines, and the like, and therefore the ignition timing is likely to fluctuate. Thus, it is difficult to accurately set the estimated knocking occurrence ignition timing.
Also, for example, when deposits are formed in the engine due to unburned fuel, blow-by gas, lubricant oil, and the like, and the degree of formation of deposits is high, it is not appropriate, to use the learning value without correction. As the engine is used, deposits are gradually formed in an intake port, an intake valve, a piston, and the like. When the degree of formation of deposits is increased, the substantive volume of a combustion chamber is decreased, and therefore, a cylinder pressure at the time of combustion is increased. As a result, knocking is likely to occur. In the publication No. 2005-147112, it is described that a ratio learning process is executed taking into account the influence of formation of deposits in the ignition timing control. However, as the load of the engine becomes lower, the influence of deposits is more notable. Therefore, when the point indicating the operating state of the engine is in the low load region, even if the ratio learning process is executed, it is difficult to accurately set the estimated knocking occurrence ignition timing.
Also, as described above, when the estimated knocking occurrence ignition timing cannot be accurately set a determination as to whether knocking occurs (hereinafter, referred to as “knocking determination”) may be adversely influenced. That is, when knocking gradually occurs, it can be accurately determined that knocking occurs. However, because the estimated knocking occurrence ignition timing cannot be accurately set, if the operating state of the engine suddenly changes, and unexpected knocking at a high level (extremely high-level knocking) occurs, it may not be possible to accurately determine that knocking occurs.
The knocking determination may be performed by comparing a peak hold value of an output from a knock sensor (shown by the chain line in FIG. 11) with a background signal and that is assumed to be a normally-distributed signal (shown by the dashed line in FIG. 11). In this case, when the level of the output from the knock sensor is equal to or above a knocking determination level (Th1 it FIG. 11), and a difference between the output from the knock sensor and the background signal is equal to or above a predetermined determination value, it is determined that knocking occurs. However, in this case, when the level of the background signal increases, for example, due to occurrence of unexpected extremely high-level knocking (as shown by the solid line in FIG. 11), the knocking determination level also increases to a higher level (Th2 in FIG. 11). When the knocking determination level increases, it may be determined that knocking does not occur even if knocking actually occurs. As a result, it is not possible to appropriately retard the ignition timing, and knocking may continue to occur.